Walkie-talkie type services have long proved popular amongst users who wish to communicate brief messages quickly between one another. Conventionally, such services have been provided by two-way portable radios which utilise a dedicated part of the radio spectrum, but which only allow users to communicate with a small group of pre-selected users who utilise similar terminals and who are within range of the relatively short operating range of the radios. More recently, services have been introduced into the United States which piggy-back on the existing cellular telephone infrastructure. However, these services have been proprietary in nature and have not allowed users to communicate between different operator networks.
In an attempt to broaden the use of walkie-talkie type services, an industry grouping known as the Open Mobile Alliance (www.openmobilealliance.org) has been established with the aim of standardising suitable protocols which will allow inter-network operability for Warlike-Talkie services offered over cellular networks. The service established by the various standards is known as Push to talk Over cellular (PoC). PoC proposes that associated speech data will be transported over a packet switched access network. In the case of GSM and UMTS, this will be the general packet radio service (GPRS) access network. In other network architectures, analogous packet switched access networks will be utilised for transporting talk data. Push to Talk services may also be offered over circuit switched access networks, although this is not the preferred option.
The Push to talk over Cellular (PoC) system is typically implemented on GSM/GPRS networks and which makes use of the IP Multimedia Subsystem (IMS) standardised by the 3rd Generation Partnership Project to facilitate the introduction of advanced data services into cellular networks, and in particular of real-time multimedia services. The IMS relies upon the Session Initiation Protocol (SIP) which has been defined by the Internet Engineering Task Force (IETF) for the setting up and control of multimedia IP-based sessions. A PoC server is located within the IMS or is attached thereto, and implements the functionality for setting up and controlling PoC sessions.
Existing push-to-talk (PTT) and conferencing systems typically use a control mechanism to grant one of the users the right to speak while other users in the communication are denied such right and are in listening mode. Such control mechanism is typically referred to as floor control, talker arbitration, talk burst control, etc. For example, the Open Mobile Alliance is currently working on a specification of Push-To-Talk over Cellular (PoC) system, which includes Talk Burst Control Protocol (TBCP).
To request the right to speak on behalf of the user the terminal typically sends a request message to the controller. The controller typically responds either granting or rejecting the request. The controller typically restricts the time the user is allowed to talk, typically by starting an allowed talk timer when it grants the request, and uses some mechanism to interrupt the user, typically by sending a revoke message to the user's terminal or by simply not forwarding the user's media. The user who is interrupted by the controller is typically penalised by the controller in some way, e.g. by not granting the user the right to speak for a certain period of time.
The typical operation of a PTT system in this regard is depicted in the FIG. 1 of the accompanying drawings.
Note that the messages depicted here do not refer to a particular protocol or implementation but are used to depict the concept of transferring the information between the terminal and the controller.
OMA-PoC User Plane specification (Open Mobile Alliance, PoC User Plane Version, Candidate Version 1.0—28 Apr. 2005, OMA-TS_PoC-UserPlane-V1—0-20050428-C) with the Talk Burst Control Protocol is a good example of these mechanisms. TBCP state machines of the terminal and the controller for the basic operation are provided respectively in FIGS. 2 and 3 of the accompanying drawings. FIG. 2 shows an OMA PoC Client state transition diagram for basic operation. FIG. 3 shows an OMA PoC Server state transition diagram for normal Talk Burst operation to the PoC Client. An OMA-PoC encoding of the TB_Granted message (TBCP Talk Burst Granted message) is provided in FIG. 4 of the accompanying drawings. Another example is the Binary Floor Control Protocol (BFCP) that is currently being specified by the IETF XCON Working Group (Internet Engineering Task Force, XCON Working Group, The Binary Floor Control Protocol (BFCP), draft-ietf-xcon-bfcp-04.txt).